There is a relatively new class of optical mice and other optical navigation input devices for computing applications. These optical devices facilitate tracking input movements on specular navigation surfaces such as glass tabletops which do not have substantial texture for imaging. In general, these optical navigation input devices rely on light scattered by small particles and scratches. This scattered light slightly increases the angular range of the otherwise collimated specular reflection off the glass surface. By capturing the scattered light off axis of the reflected beam using an offset imaging aperture (i.e., the imaging aperture is offset relative to the main intensity of the specular reflection of the incident light), images of such scattering sites can be projected on a pixel array in a sensor, which can then be used to determine the motion of the mouse relative to the tabletop.
In the process of tracking on a very smooth surface with relatively few features, scattered light from such features is typically collected near the angle of specular reflection of the optical beam used to probe the surface. However, the scattering from a very smooth surface falls off rapidly in relation to a distance from the specular reflection. Hence, the scattered light is in close proximity to the specularly reflected beam in the plane of the imaging aperture of the optical system. However, the specularly reflected beam is usually much brighter (i.e., has a higher intensity) than the scattered light which is used to create the optical images. So an illumination aperture may be used to adjust the outer intensity profile of the incident light beam at an appropriate radius from the center of the light beam. This adjustment helps to avoid having the peripheral portions of the intensity profile of the specularly reflected light beam incur upon the imaging aperture and create excessive noise for the scattered light beam.
Typical laser beams leave the collimating optics of the optical system with a circular cross section at some angle of incidence relative to the navigation surface. Since the cross section of the incident laser beam is circular, the resulting shape of the light beam on the navigation surface is elliptical, depending on the angle of incidence of the light beam. The eccentricity of the elliptical shape is based on the following equation:
  e  =      1          cos      ⁡              (                  q          inc                )            where e is the eccentricity, and qinc is the angle of incidence of the incident light beam.
FIG. 1 shows one example of a relationship between the circular cross section of the incident light beam and the elliptical shape of the illuminated spot on the navigation surface. In particular, the circular cross section of the light beam is indicated at plane A, where the circular cross section has a constant radius, r. In contrast, the elliptical shape of the light beam is indicated at plane B, which is parallel to the navigation surface and at an angle relative to plane A. The elliptical projection has two different dimensions, a and b, along corresponding major and minor axes. Although the b-dimension of the elliptical shape at plane B may be the same as the r-dimension of the circular cross section at plane A, the a-dimension of the elliptical shape is greater than the r-dimension of the circular cross section because of the angle between planes A and B. Similar elliptical shapes result at the illumination and imaging apertures, as well as the image sensor, if these devices are oriented substantially parallel to the navigation surface.
Since the shape of the light beam is elliptical in a plane parallel to the navigation surface, it can be difficult to aperture and/or generate an image of the light beam. One conventional solution is to tilt the apertures and image sensor to correlate to the angle of incidence of the light beam. However, this approach typically results in sophisticated and costly packaging and alignment because of the precision with which the light beam is tailored to achieve proper truncation. In addition, the elliptical shape of the beam at the imaging aperture makes it difficult to place the specularly reflected light beam in close proximity to the imaging aperture without encroaching on the imaging aperture. Also, conventional collimators generally involve input and output optical lens surfaces, which result in multiple reflections within the collimator. These multiple reflections can lead to a scattered “halo” of light around the main beam, which can also encroach on the imaging aperture and, hence, result in further noise problems.